Obsidian roof
The obsidian roof is a large platform of obsidian above the spawn region of the Overworld. It is built at a build limit (Y=255) for maximum impact.
The roof has been expanded over the years by many different players, groups, and initiatives, from just a small roof over 0,0 to its current status, which is a massive obsidian platform, mostly square in shape, of over 4.2 million blocks.
Obsidian Sky Group
The first attempt at making an obsidian roof at spawn was done by the group Obsidian Sky. Their reason for doing this was so that it would make spawn more difficult by having newfags spawn under the roof, thus making it dark and harder to see for vanilla players without cheat clients. Eventually, the Obsidian Sky group got bored and only completed random patches of their goal which was a square from -500 to +500.
The circle
After building The Masonic Eclipse, the software was repurposed to build a circle at spawn. The radius was 710 blocks so that it would extend the corner of the previous square (because 710 ≈ sqrt(2)*500). It was built in early 2020 and was about 800,000 obsidian placed to bring it from a "square" to a circle to invoke irony amongst the community (as Minecraft has no circular objects).
The 2b2t atlas map also uses the map as of when the circle was full. The black circle at the center isn't a mistake, it's an actual disc of obsidian in that shape.
Infinity Incursion
Another extension to the obsidian roof over spawn was started by the Infinity Incursion in mid-2020. This attempt utilized a recently added "autobuild" module in Salhack which allowed semi-automatic platform building. Members of the II got to work, and within a few weeks, the roof was created. This extension was a square from -1025 to +1025 on both axes.
Linked Horizon Sky
The most recent extension to the obsidian roof was built by the Linked Horizon group in the summer of 2021. After the completion of the Linked Horizon Wall, the group wanted to begin a new project, so players Nonus and Futsin1 began discussing ideas about what they should do. Eventually they came to the obvious answer; building a ceiling above their previous Linked Horizon Wall. The ceiling would be in the shape of a circle, with a radius of 2048 blocks. IronException calculated the amount of blocks needed, and ThebesandSound provided the necessary obsidian he had obtained previously from the Chunk Dupe. Using scaffold, anti-no fall and eventually Lambda highway tools, the group began expanding the roof, and finally after 6 months, on January 6th, 2022, they placed the final blocks on the roof, finishing the project. Along with the highways, and the SpawnMason Logo, the Linked Horizon Structure is one of the largest structures on 2b2t by volume.